1. Field of the Invention
The present invention relates to a twin-screw continuous kneading apparatus suitable for efficiently kneading high-polymer resin materials, such as plastic and rubber mixed with additives, to obtain a plastic compound and a rubber compound. The present invention also relates to a kneading method using the twin-screw continuous kneading apparatus.
2. Description of the Related Art
Generally, composite resin materials, such as a plastic compound and a rubber compound, are produced by supplying a powdery additive, such as talc, into a barrel of a kneading extruder, and extruding a high-polymer resin material to be kneaded downstream using a kneading screw, which is inserted in the barrel, while kneading the high-polymer resin material together with the additive. The extruded material is then shaped into products by a post-treating apparatus such as a granulating machine.
A twin-screw continuous kneading apparatus for producing such composite resin materials comprises a barrel having a pair of left and right kneading chambers formed therein in communication with each other, a pair of left and right kneading screws rotatably inserted in the kneading chambers, a first supply unit constituted by, e.g., a screw feeder connected to the upstream end of the barrel and supplying a material to be kneaded into the kneading chambers, and a second supply unit constituted by, e.g., a side feeder connected to a midstream portion of the barrel and supplying an additive into the kneading chambers. A kneading zone for kneading and mixing the additive into the kneaded material is defined along the kneading screws in an area downstream of the second supply unit.
In the kneading zone, rotor segments each having a tip clearance, which allows the kneaded material to positively pass through it, are employed so that high shearing forces are imparted to the kneaded material even under low temperatures. Also, to control a filling rate of the material, a plurality of kneading disks having phases shifted in a direction to push back the material or screw segments having flights twisted in a direction to push back the material are usually provided in the kneading zone.
However, providing those kneading disks or screw segments acting to push back the resin flow in the kneading zone immediately downstream of a point, at which the additive is added to the resin material, may cause the problem in that the filling rate of the material is locally increased in the kneading zone to such an extent as impeding a gas purge toward the downstream side, and the additive cannot be supplied through the side feeder any more. In that event, the feeding capability is reduced and the stable operation is no longer continued.
One conceivable method for overcoming the above-mentioned problem is to construct the kneading zone, which is positioned immediately downstream of the addition point of the additive, only by rotor segments having flights twisted in a direction to feed the resin material without providing the kneading disks and the screw segments.
However, the tip clearance of the rotor segment is set, as mentioned above, to be comparatively larger than those of other kneading segments. Accordingly, if the kneading zone positioned immediately downstream of the addition point of the additive is entirely constructed using such conventional rotor segments, the kneaded material passing through the tip clearances is more likely to adhere to inner wall surfaces of the kneading chambers in the form of layers and to reduce the effective diameter of each kneading chamber. This may result in a reduced throughput and insufficient kneading of the resin material with the mixed additive.
Also, employing the kneading zone, in which the rotor segments having the tip clearances comparatively larger than those of other kneading segments are successively arranged, raises another problem in that a kneading screw is more susceptible to a mechanical damage because of an increased deflection during the rotation thereof, and the kneading screw cannot be rotated at high speeds. This also impedes an improvement of the throughput.
The above-described problems arise not only in the case of mixing a powdery additive into a material to be kneaded, but also in the case in which the material to be kneaded is itself a powdery material having a very small bulk density.
In view of the state of the art set forth above, it is an object of the present invention to provide a twin-screw continuous kneading apparatus and a kneading method using the twin-screw continuous kneading apparatus, which can stably supply a powdery additive or a powdery material to be kneaded without reducing the production capability, and can greatly increase the throughput of a high-quality compound.
To achieve the above object, a twin-screw continuous kneading apparatus according to the present invention comprises a barrel having a pair of kneading chambers formed therein in communication with each other; a pair of kneading screws rotatably inserted in the kneading chambers; a first supply unit connected to an upstream end portion of the barrel and supplying a material to be kneaded into the kneading chambers; a second supply unit connected to the upstream end portion or a midstream portion of the barrel and supplying an additive into the kneading chambers; an additive feed zone provided by a part of each of the kneading screws corresponding to the second supply unit, the additive feed zone being made up of screw segments; a downstream feed zone provided by a part of each of the kneading screws downstream of the additive feed zone, the downstream feed zone being made up of screw segments; and an additive mixing zone provided by a part of each of the kneading screws downstream of the additive feed zone and between the additive feed zone and the downstream feed zone, the additive mixing zone being made up of rotor segments arranged successively in an axial direction of the kneading screw. A segment group constituting the additive mixing zone satisfies conditions (a) and (b) given below: (a) the segment group comprises only feed segments having flights twisted in a direction to feed the kneaded material downstream with rotation of the kneading screw, or comprises the feed segments and neutral segments having flights parallel to the axial direction of the kneading screw, and (b) each of the rotor segments making up the segment group has flights formed by arranging a high-height tip portion providing a comparatively small tip clearance relative to an inner wall surface of the kneading chamber and a low-height tip portion providing a comparatively large tip clearance alternately in the axial direction in succession.
With the arrangement set forth above, the segment group constituting the additive mixing zone, which is made up of a plurality of rotor segments arranged successively in the axial direction of the kneading screw, comprises only the feed segments or comprises the feed segments and the neutral segments. Thus, any segments, which develop an action to push back a flow of resin (kneaded material) upstream, are not present in the additive mixing zone. It is therefore possible to prevent the filling rate of the kneaded material from being locally increased in the additive mixing zone to such an extent that degassing toward the downstream side would be impeded. As a result, the feeding capability is avoided from deteriorating due to reduced supply of the additive from the second supply unit, and hence the stable operation is ensured.
Also, with the above arrangement, the flights of each rotor segment of the segment group constituting the additive mixing zone are formed by arranging the high-height tip portion providing the comparatively small tip clearance relative to the inner wall surface of the kneading chamber and the low-height tip portion providing the comparatively large tip clearance alternately in the axial direction in succession. Therefore, mixing of the additive into the kneaded material is accelerated when the kneaded material passes through the comparatively large tip clearance provided by the low-height tip portion. In addition, even if the kneaded material adheres to the inner wall surfaces of the kneading chambers when passing through the comparatively large tip clearance provided by the low-height tip portion, the adhered kneaded material is scraped off from the inner wall surfaces of the kneading chambers by the high-height tip portion that provides the comparatively small tip clearance. It is hence possible to prevent the effective diameter of each kneading chamber from being reduced due to adhering of the kneaded material in the form of a layer onto the inner wall surface of the kneading chamber, while improving the effect of mixing the additive into the kneaded material. As a result, the throughput of a compound can be greatly improved.
In the present invention, the rotor segments making up the segment group of the kneading zone may be each of the three-flight type having three projected flights, or of the two-flight type having a pair of flights radially projecting in opposite directions. From the viewpoint of increasing the throughput, the two-flight type is more preferable because it is able to provide a larger effective cross-sectional area in the kneading chamber.
As compared with the three-flight rotor segment, the two-flight one has a smaller transverse cross-sectional area, generates a relatively large deflection during rotation at high speeds, and is more susceptible to a mechanical damage. Using the two-flight rotor segment, therefore, accompanies a contradiction that, in spite of providing a larger effective cross-sectional area in the kneading chamber, the two-flight rotor segment cannot improve the throughput to such an extent as expected, because the kneading screw cannot be rotated at high speeds.
In the present invention, therefore, when each of the rotor segments making up the segment group is of the two-flight type having a pair of flights radially projecting in opposite directions, the flights are preferably located such that in an axial range where one of the flights is formed as the high-height tip portion, the other flight is formed as the low-height tip portion, and in an axial range where one of the flights is formed as the low-height tip portion, the other flight is formed as the high-height tip portion.
With that arrangement, since both of the paired flights are never formed as the low-height tip portions in any axial range, a reduction in cross-sectional area of the rotor segment caused by the formation of the low-height tip portions is minimized. Consequently, the rotor segment is less susceptible to a mechanical damage even with the kneading screw rotated at high speeds, and the rotational speed of the kneading screw can be increased in spite of using the two-flight rotor segment. The throughput of a compound can be hence improved.
In the present invention, preferably, the pair of kneading screws are of the meshing type in which flights of the kneading screws mesh with each other inside the barrel.
With that arrangement, one of the kneading screws scrapes off the kneaded material in a molten state from the other to prevent the kneaded material from remaining adhered to each kneading screw. As a result, the kneaded material can be avoided from residing too long in the kneading chambers and from deteriorating.
In the present invention, preferably, all of the rotor segments making up the segment group constituting the additive mixing zone have the same cross-sectional form except for the tip portions and are joined to each other continuously such that no steps are produced between every adjacent rotor segments in the axial direction.
When employing the kneading screws of the meshing type, if the rotor segments making up the segment group constituting the additive mixing zone have much different cross-sectional forms or they are joined to each other discontinuously, a flow of the kneaded material would be impeded by steps produced at joints between the adjacent segments, and the kneaded material would be more likely to reside too long in the kneading chambers. Those drawbacks can be eliminated with the above arrangement.
In the present invention, preferably, a tip clearance provided by the low-height tip portion is set to a range of 0.015D to 0.150D, and a tip clearance provided by the high-height tip portion is set to a range of 0.001D to 0.020D, in which D is an inner diameter of the kneading chamber. By so setting the tip clearances, the additive can be effectively mixed into the kneaded material.
Further, the kneading apparatus of the present invention may be constructed such that the second supply unit is connected to the midstream portion of the barrel, the apparatus further comprises a kneaded material feed zone provided by a part of each of the kneading screws corresponding to the first supply unit, the kneaded material feed zone being made up of screw segments, and each of the kneading screws includes a first kneading zone located between the kneaded material feed zone and the additive feed zone and kneading the material to be kneaded, which is supplied by the first supply unit.
As an alternative, the kneading extruder may be modified such that the second supply unit is connected to the upstream end portion of the barrel, and the additive feed zone serves also as a feed zone for the material to be kneaded, which is supplied by the first supply unit.
The kneading extruder may further comprise a gate device having a pair of upper and lower gate plates, which are movable toward and away from the kneading screws, the gate device being located at a downstream end of the additive mixing zone.
In addition, the present invention provides a kneading method using a twin-screw continuous kneading apparatus, wherein an additive supplied by the second supply unit is mixed in the additive mixing zone into a material to be kneaded, which is supplied by the first supply unit.
The kneading method of the present invention may be applied to production of a composite resin material such as a plastic compound or a rubber compound. In such a case, preferably, the kneaded material supplied by the first supply unit is in the form of pellets made of a synthetic resin, and the additive supplied by the second supply unit is an additive made of a reinforcement powder.
According to another aspect, the present invention provides a twin-screw continuous kneading apparatus comprising a barrel having a pair of kneading chambers formed therein in communication with each other; a pair of kneading screws rotatably inserted in the kneading chambers; a first supply unit connected to an upstream end portion of the barrel and supplying a material to be kneaded into the kneading chambers; a first feed zone provided by a part of each of the kneading screws corresponding to the first supply unit, the first feed zone being made up of screw segments; a downstream feed zone provided by a part of each of the kneading screws downstream of the first feed zone, the downstream feed zone being made up of screw segments; and a first kneading zone provided by a part of each of the kneading screws downstream of the first feed zone and between the first feed zone and the downstream feed zone, the first kneading zone being made up of rotor segments arranged successively in an axial direction of the kneading screw, wherein a segment group constituting the first kneading zone satisfies conditions (a) and (b) given below: (a) the segment group comprises only feed segments having flights twisted in a direction to feed the kneaded material downstream with rotation of the kneading screw, or comprises the feed segments and neutral segments having flights parallel to the axial direction of the kneading screw, and (b) each of the rotor segments making up the segment group has flights formed by arranging a high-height tip portion providing a comparatively small tip clearance relative to an inner wall surface of the kneading chamber and a low-height tip portion providing a comparatively large tip clearance alternately in the axial direction in succession.
The present invention is applicable to not only the case of mixing a powdery additive into a material to be kneaded, but also a twin-screw continuous kneading apparatus for handling the material to be kneaded, which is itself a powdery material having a small bulk density, and a kneading method implemented using the kneading extruder. In the latter case, the provision of the second supply unit constituted by, e.g., a side feeder is not necessarily required.